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Performance of the 305m diameter Arecibo radio/radar telescope has been significantly improved by the addition of a screen 16m high and 1km long around the perimeter of the reflector, to reduce thermal noise produced by radiation from the ground. Installation of the screen was completed in August, 1993 as the first phase of the Gregorian Upgrading Program at Arecibo. Measurements of system temperature with an existing 21 cm line feed, carried out with and without the screen in place, show an improvement of 28K at the maximum zenith angle of $20\deg$. The measured improvement agrees with predicted performance.
The Gregorian subreflector, now under construction, will move the illumination pattern 15m downhill, further reducing vignetting noise. With this offset, the measurements project a further 14K noise reduction. The system temperature increase with zenith angle for the 21 cm line feed system, prior to installation of the ground screen, was 47K. In contrast, the Gregorian system temperature is expected to increase by no more than 5K at maximum zenith angle, an order of magniture improvement.
The National Astronomy and Ionosphere Center is operated by Cornell University under Cooperative Agreement with NSF. Funding for the Gregorian Upgrading is provided jointly by NSF and NASA.
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